This isn’t possible – you’ll need to register each subdomain on dev.twitter.com that you’ll be using. How many subdomains are you looking to support? It’s possible that a server-side integration might make more sense for you.

The application is a web hosting platform, with each client having their own subdomain. There are presently around 1000 subdomains, but there are more daily.
Is there any way to create a session using server-side auth that @Anywhere can use?

In this scenario, it’d likely be best to either stay away from @Anywhere completely or require that each subdomain-holder register their own @Anywhere keys tied to their personal sub-domain. The security model of @Anywhere doesn’t match well with multi-tenant hosting platforms like this. Are there specific features in @Anywhere you’re especially wanting to offer?